Significant differences of NH3-SCR performances between monoclinic and hexagonal WO3 on Ce-based catalysts

Abstract

WO₃ has different crystalline structures, including monoclinic and hexagonal phases. Monoclinic WO₃ (W-m) is the most common crystal phase in NH₃-SCR catalysts. Hexagonal WO₃ (W-h) with empty hexagonal and trigonal tunnel structures exhibits excellent acidity and reducibility and can provide abundant catalytically active sites, but it is hardly applied in the NH₃-SCR reaction. Herein, the effects of different WO₃ phases on the NH₃-SCR performance of Ce-based catalysts are systematically investigated. The SCR activity of CeO₂ is both evidently improved by the addition of W-h and W-m. It is noteworthy that CeO₂–WO₃-hexagonal (Ce–W-h) exhibits a more excellent NO_x removal efficiency than CeO₂–WO₃-monoclinic (Ce–W-m) in the whole temperature window tested, and T₅₀ and T₉₀ of Ce–W-h are 51 and 111 °C lower than those of Ce–W-m, respectively. Importantly, compared with that of W-m, the special open structure of W-h not only facilitates NH₃ adsorption and the diffusion of oxygen to improve the acidity and reducibility of the catalyst, but also is favorable for electron transfer between W-h and Ce to boost the interaction between W-h and Ce. Besides, the active plane (001) on W-h/Ce–W-h can provide abundant unsaturated W⁶⁺ and Brønsted acid sites. Accordingly, the Ce–W-h catalyst displays a more impressive NH₃-SCR performance than Ce–W-m. W-h is a better choice to modify CeO₂ to improve its SCR performance than W-m. This work provides a profound understanding of the intrinsic effect of the crystal phases of WO₃ on NH₃-SCR performance.